Acidic sea will spoil fish navigation

Nemo, the lovable clownfish of movie fame, may be unable to find his way home as the world’s oceans acidify due to human carbon emissions.

Fish lose the sense of smell that guides them home when sea water becomes more acidic – with potentially devastating consequences for sea life, an international team of scientists has discovered.

Like many reef and coastal fish, Nemos are swept off their home reef into the open ocean as tiny babies and use their acute sense of smell to find their way back again.

Now, say the researchers, every time we start a car or turn on the lights, a third or more of the carbon dioxide we emit ends up in the ocean, turning its waters imperceptibly but inevitably more acidic.

The researchers tested clownfish raised in normal and slightly more acidic seawater to see what effect it had on their sense of smell. To their dismay they found that the fish, normally acutely sensitive to different smells in the water, became confused when the water they were raised in was more acidic than usual.

“We found the baby fish were strongly attracted to scents they normally avoided at the sort of levels of ocean acidity that could occur by 2100 – and they no longer responded to scent cues at all when the acidity rose to the sorts of levels likely by the second half of that century if nothing were done to curb carbon emissions,” says lead author Dr Philip Munday of the ARC Centre of Excellence for Coral Reef Studies.

“This is a disturbing finding, because the tiny larvae of many coastal fish probably rely on scent cues in the water to help locate adult habitat.

“Any disruption to their ability to navigate could have far-reaching implications for the future of these fish populations.”

Ocean acidification, caused by the absorption of human-released carbon dioxide (CO2) at the ocean surface, is now recognized as a serious threat to marine ecosystems, the researchers from James Cook, Moscow and Oslo universities say.

“At least 30 per cent of the human-generated CO2 released into the atmosphere in the past 200 years has been absorbed by the oceans, causing ocean pH to decline (ie to acidify) at a rate around 100 times faster than at any time in the past 650 000 years.

“Global ocean pH is estimated to have dropped by 0.1 units since preindustrial times and is projected to fall another 0.3-0.4 units by 2100 due to current and future CO2 emissions.”

“Orange clownfish – or nemos – mostly live on oceanic reefs surrounding vegetated islands and research has shown that baby fish can discriminate between seawater from reefs surrounding vegetated islands versus seawater from reefs without islands. This is what enables many of the baby clownfish to find their way back to the reef where they were hatched”, says Dr Jones, one of the studies authors.

In normal sea water nemos are strongly attracted to scents from anemones on their home reefs and tropical rainforest trees on nearby land – but avoid the smell of swamp trees or tropical grasses.

Nemos reared in slightly more acidic water were attracted to a range of environmental smells, including those they normally avoided and that indicated a habitat less suited to their needs. Nemos reared in seawater at pH 7.6 showed no response to scent cues at all. The scientists conclude that prolonged exposure of the larvae to lower pH water has somehow impaired their ability to smell – and so find their way to the home reef.

“Our results show for the first time that elevated CO2 and reduced seawater pH that could occur early next century in the world’s oceans can affect the behavioural decisions of marine organisms during critical stages of their life-history. In this case, acidification disrupted the olfactory mechanism by which clownfish larvae discriminate between cues that may be useful for locating suitable adult habitat and other cues that could lead larvae to unsuitable settlement sites.

“Disruption to this process would have significant consequences for the replenishment of adult populations and could lead to declines of many coastal species,” they conclude.